The present invention relates to commercial mass production electroplating of gold and more particularly to decorative gold plating solutions utilized in said production.
Decorative gold electroplating came into vogue at first in France during the 19th Century pursuant to the discovery by Glassford Napier (Philosophical Magazine 25, 61 (1844)) that gold dissolves anodically in potassium cyanide to give KAu(CN).sub.2, which assays approximately 68% Au. That salt is called potassium gold cyanide, or by those skilled in the art, simply PGC. PGC was the solid utilized in the first electroplating solutions. They were extremely toxic, containing large quantities of alkaline cyanides to foster conductivity and stability. The deposits were rich 24 karat gold. Gold concentrations were about one to two troy ounces per gallon of gold. Subsequently it was discovered that by adding various metal cyanide salts to the solution, various colors could be achieved. For example, 18 and 14 karat colors were obtained by adding nickel cyanide at various concentrations, and Vermeil green gold by adding silver cyanide.
As time progressed, the huge concentrations of cyanides were diminished as it was learned that various mixtures of alkali phosphates, carbonates and hydroxides could partially replace the cyanides. PGC was sometimes replaced with the sodium gold salt. Nevertheless, the current state of the art prefers PGC. This is exemplified by the current Metal Finishing Guidebook Directory for 1995. It suggests gold concentrations in alkaline solutions should vary between one to one and a half troy ounces per gallon and contain 240-400 g. of potassium cyanide per gallon for bright gold deposits. Such a solution requires a minimum inventory for a ten gallon bath at all times of approximately $4,000 based on current gold prices. The bath is highly toxic and can only be operated in conformity with environmental statutes with special cyanide waste treating systems. On the other hand, Robert Duva and Edwin Rinker, invented acid gold electroplating solutions, exploiting the fact that PGC is a highly stable gold complex and can exist at pH values in the presence of Citric acid as low as 2.5, but recommended commercial baths closer to 4.0 for which they were awarded U.S. Pat. No. 2,905,601 in 1959. These solutions were used extensively in decorative plating and required less gold. At the time they were introduced by the patent assignee, Sel Rex Corporation, gold was selling at $35 an ounce, so that a ten gallon bath at the time would require about $400 of PGC at an Au concentration of one troy ounce per gallon compared to today's aforementioned cost of $4,000. With the rise of gold costs and the financial pressures for reduced inventories, it was found that the Citric acid baths could be sustained in production at operational concentrations of four pennyweights of gold per gallon. Nevertheless, in mass production involving opportunistic production contractors and mass producers of decorative goods, such as costume jewelry, where 100 gallon baths are the rule and not the exception, this would translate into $8,000 of inventory per 100 gallon bath.
When both types, the alkaline and acid gold solutions are employed for decorative plating, there reaches a point where the solutions are contaminated and must be discarded. The residual gold in the solution is refined for recovery, but at a fixed loss. New electrolytes must now be purchased for restarting the bath. Accordingly, these baths have a "put through" limit, that is--how many ounces of PGC can be put through the bath as gold is consumed until the bath is exhausted for replenishment. The salts utilized for the acid baths are more expensive and somewhat proprietary compared to those for the alkaline baths. The acid baths can also be utilized at neutral pH's. The acid type golds are harder than the alkaline, which are soft. Unlike the aforementioned state of the art, in opportunistic plating, one tries to achieve the maximum specification of gold at the least operating and inventory maintenance costs. This includes a minimum deposit of gold, called a flash deposit, where only appearance is required, a minimum soft gold deposit from an alkaline gold solution. On the other hand, if one wants durability, the more expensive acid type bath is used for flash deposits. Now both types of solutions require metal salt additions to achieve various colors other than 24 kt. gold and concentration maintenance. The solutions also must be chemically adjusted for production runs of both racked and barrel plated goods.
From the aforesaid factors one can define the goals of an effective and opportunistic gold electroplating solution, the subject of this invention, as follows:
1. Ecological PA1 2. Low gold concentration PA1 3. Maximum "put through" PA1 4. Durability of gold deposit PA1 5. Metal salt-free non-24 karat color maintenance Following these goal guidelines, the first two goals were addressed. The rationale was to develop a plating solution without free cyanide. Accordingly, the only cyanide was to be complexed cyanide of PGC. The next goal was to achieve a minimum operating Au concentration never before achieved in mass production plating, i.e. one-half pennyweight per gallon. This was achieved. Thereafter, the goal of a "put through" of 20 times (20.times.) that concentration was attempted and was achieved. Accordingly, the total amount of gold added to the bath over its life was ten pennyweights or one-half troy ounce per gallon. The residual CN concentration was no greater than 500 PPM. After these achievements, pH adjustments of sundry prototype solutions, led to a variety of flash golds with exceptional durability. Finally a wide range of colors was achieved with these solutions ranging from rich 24 karat to light 18 and 14 karat colors dependent on a wide parameter voltage-temperature variability grid. All of these achievements were done on a laboratory scale utilizing beakers and Hull cells, with platinum clad anodes. Thereafter, the best solutions were evaluated in five and ten gallon sections of a compact mini production plating console. After achieving success, a two-year actual production research and development program was undertaken with various mass production gold electroplaters in the Providence, R.I./Attleboro, Mass. area, known as a major center for decorative goods production. It was under these actual production conditions where the baths were tested and perfected for particular results. It was found invariably that the "put through" life of a particular plating bath was substrate as well as chemical dependent. PA1 1. Hydroxyacetic acid and acetic acid mixture PA1 2. Hydroxyacetic acid PA1 3. Acetic acid
The final PGC gold functional electrolytes described in this invention were based on the following three types of ingredients: a primary buffer, salt of acetic acid and/or an alpha hydroxy C.sub.n H.sub.n derivative salt; and in some cases a secondary buffer, i.e. an alkaline phosphate; and in some cases a chelating agent.